Good morning, Jeff! My application was also received on December 6th, and I haven't heard anything either. No letter yet, and my eCAS has not been updated. And, according to the Processing Times: Family Sponsorship page they were working on December 6 applications on Wednesday.
My thinking is that December 6 was a Monday, so perhaps they had more applications come in that day than usual (ones that had piled up over the weekend), so maybe it's taking them longer than usual to get through it all.
Also, we haven't heard from anyone who submitted theirs after the 6th who has received approval.
I know that the waiting is hard, but hold on.
I'm hoping for the best, and also wishing you the best!